Major Description
As detailed in this blog post:
http://www.cloudera.com/blog/2009/02/10/multi-host-secondarynamenode-configuration/
if dfs.http.address is not configured, and the 2NN is a different machine from the NN, the 2NN fails to connect.
In SecondaryNameNode.getInfoServer, the 2NN should notice a "0.0.0.0" dfs.http.address and, in that case, pull the hostname out of fs.default.name. This would fix the default configuration to work properly for most users.